RE: [PATCH linux-next v3 0/6] vdpa: enable user to set mac, mtu

[Parav Pandit via Virtualization]

> From: Eli Cohen <e...@nvidia.com>
> Sent: Monday, August 9, 2021 11:28 AM
> 
> On Mon, Aug 09, 2021 at 08:42:58AM +0300, Parav Pandit wrote:
> >
> >
> > > From: Eli Cohen <e...@nvidia.com>
> > > Sent: Monday, August 9, 2021 10:51 AM
> > >
> > > On Mon, Aug 09, 2021 at 11:07:50AM +0800, Jason Wang wrote:
> > > >
> > > > 在 2021/8/6 下午4:55, Parav Pandit 写道:
> > > > >
> > > > > > From: Michael S. Tsirkin <m...@redhat.com>
> > > > > > Sent: Friday, August 6, 2021 2:12 PM
> > > > >
> > > > > > >   enum vdpa_attr {
> > > > > > > @@ -33,6 +34,16 @@ enum vdpa_attr {
> > > > > > >           VDPA_ATTR_DEV_MAX_VQS,                  /* u32 */
> > > > > > >           VDPA_ATTR_DEV_MAX_VQ_SIZE,              /* u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_MACADDR,          /*
> > > binary */
> > > > > > > + VDPA_ATTR_DEV_NET_STATUS,               /* u8 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_MAX_VQP,          /*
> > > u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_MTU,              /* u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_SPEED,            /* u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_DUPLEX,           /* u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_RSS_MAX_KEY_LEN,  /* u8
> > > */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_RSS_MAX_IT_LEN,   /*
> > > u16 */
> > > > > > > + VDPA_ATTR_DEV_NET_CFG_RSS_HASH_TYPES,   /*
> > > u32 */
> > > > > > > +
> > > > > > >           /* new attributes must be added above here */
> > > > > > >           VDPA_ATTR_MAX,
> > > > > > >   };
> > > > > > The point is to try and not reinvent a dedicated vpda
> > > > > > interface where a generic one exits.
> > > > > > E.g. for phy things such as mac speed etc, I think most people
> > > > > > are using ethtool things right?
> > > > > As you know vdpa is the backend device for the front-end
> > > > > netdevice
> > > accessed by the ethtool.
> > > > > vdpa management tool here is composing the vdpa device.
> > > > >
> > > > > For example creator (hypervisor) of the vdpa devices knows that
> > > > > a guest VM is given 4 vcpus, So hypervisor creates a vdpa
> > > > > devices with config space layout as, max_virtqueue_pairs = 4.
> > > > > And the MAC address chosen by hypervisor in mac[6].
> > > > >
> > > > > Guest VM ethtool can still chose to use less number of channels.
> > > > >
> > > > > Typically,
> > > > > ethtool is for guest VM.
> > > > > vdpa device is in hypevisor.
> > > > >
> > > > > How can hypervisor compose a vdpa device without any tool?
> > > > > How can it tell ethtool, what is supported and what are the defaults?
> > > >
> > > >
> > > > Reread the cover letter:
> > > >
> > > > "
> > > >
> > > > This patchset enables users to set the mac address and mtu of the
> > > > vdpa device once the device is created.
> > > >
> > > > "
> > > >
> > > > It looks to me the mechanism that introduced in the series is not
> > > > for provisioning but for post-creation configuration?
> > > >
> > >
> > > The difference is subtle. In both cases you provide configuration.
> > >
> > >
> > > >
> > > > >
> > > > > I must be misunderstanding your comment about ethtool.
> > > > > Can you please explain?
> > > >
> > > >
> > > > I guess the meaning is that, if the vDPA is assigned to guest,
> > > > it's the charge of guest to configure the MTU/MAC/RSS via the
> > > > existing management interface like ethtool/iproute2 netlink
> > > > protocol. The control virtqueue is designed for this.
> > > >
> > >
> > > I was under the impression that we want somehow to control the
> > > capablity if the guest to use arbitrary MAC addresses.
> > > If this is is not required than I think control VQ is the mechanism to 
> > > use.
> > How does the guest VM identify which unique mac address to set on this
> virtio net device when this is the only device in the VM?
> > Usually hypervisor knows what mac to set for a VM.
> 
> You don't need to know. You could use any MAC you want, if no one else is
> using it in your subnet, and everything will work fine. The point is do you
> want to allow the guest to choose its MAC. This has implications of security.
> 
Lets assume for a moment that a guest VM is able to program a MAC of netdevice 
of virtio_device of net type.
How does a VM know that a randomly chosen mac is not used in network when this 
VM doesn’t have any external connectivity?
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